The present invention relates to a ceramic electrostatic chuck with built-in heater or, more particularly, to a ceramic-made device used for heating a flat substrate material such as a semiconductor silicon wafer as a workpiece under chucking with an electrostatic attracting force in the manufacturing process of, for example, semiconductor devices which involves a step of working on the substrate at an elevated temperature.
It is sometimes the case in the manufacturing process of semiconductor devices that working on a substrate such as a semiconductor silicon wafer is performed under heating of the substrate at a high temperature by mounting the wafer on a heater. Conventional heaters used for this purpose utilize electric resistance heating with a coiled resistance wire as the resistance heater element. Such an electric heater, however, is disadvantageous in respect of the possibility of contamination of the semiconductor silicon wafer with the metallic elements as the constituents of the resistance wires. In this regard, proposals have been made for a so-called ceramic heater which is an integral device consisting of an electrically insulating ceramic base body and a thin film of an electroconductive ceramic material such as graphite formed on one surface of the ceramic base body to serve as a heater element (see, for example, Japanese Patent Kokai No. 4-124076).
It is also required in the above mentioned process of a semiconductor silicon wafer with a ceramic heater that the silicon wafer is fixed and immobilized on the heater in order to ensure accuracy of working on the silicon wafer by the use of a chucking means. Since the working process of a silicon wafer in many cases is conducted under a reduced pressure or in vacuum, traditional vacuum chucks can no longer work there so that electrostatic chucks are currently under use in an atmosphere under reduced pressure. Along with the trend in the semiconductor processes that the working temperature on a silicon substrate is increased higher and higher, ceramics are used as the material of the electrostatic chucks (see, for example, Japanese Patent Kokai No. 52-67353 and No. 59-124140). It is also a trend in recent years to use a ceramic electrostatic chuck with built-in heater, which is an integral device as a combination of a ceramic electrostatic chuck and a ceramic heater. Such a ceramic electrostatic chuck with built-in heater has a structure in which a base body of a ceramic material is provided, on one surface, with a first electroconductive layer for electric resistance heating and, on the other surface, with a second electroconductive layer to serve as the electrodes for generating an electrostatic attracting force, each of the electroconductive layers being overlaid with an insulating layer for protection. When the working temperature of such a ceramic electrostatic chuck with built-in heater is relatively low as in the etching process, the material of the insulating layer thereof can be alumina and the like (see, for example, Japanese Patent Kokai No. 59-124140) while, when the working temperature is very high as in the CVD process, a highly refractory material such as pyrolytic boron nitride and the like is used for the insulating layer thereof (see, for example, Japanese Patent Kokai No. 4-358074, No. 5-109676 and No. 5-129210).
Although, as is described in several literatures, the electrostatic attracting force of a ceramic electrostatic chuck is increased as the volume resistivity of the insulating layer is decreased, the insulating layer in an electrostatic chucking device must have a volume resistivity in the range from 10.sup.10 to 10.sup.13 ohm.multidot.cm or, preferably, around 10.sup.11 ohm.multidot.cm because, when the volume resistivity of the insulating layer is too low, eventual break of the device is sometimes caused due to a leak current.
When alumina is used as the material of the insulating layer of a ceramic electrostatic chuck working in an intermediately high temperature range of 500.degree. C. to 650.degree. C., for example, the resistivity of the insulating layer is so low that break of the device is sometimes unavoidable due to the leak current while, when pyrolytic boron nitride is used for the insulating layer, the resistivity of the insulating layer is too high to give a sufficiently high electrostatic attractive force of, for example, 100 to 500 gf/cm.sup.2.